White Cat by Holly Black

Cassel, while dreaming of a white cat, has somehow ended up on the roof of his dorm in the middle of the night. This incident is going to destroy all of the hard work he has done creating a façade of normalcy about him, so that he can just blend in with the crowd.

And Cassel has to put a lot of work into that façade for several reasons. He is actually the youngest in a family of curse workers, people who have the power to change your emotions, your memories, and your luck with just the slightest touch of their hands. And since curse work is illegal, they are all criminals. All, except for Cassel, since it appears that he is completely without the magic that makes curse work possible. But you would need to overlook that one time three years ago when Cassel killed his best friend, Lila. Or did he? And what was that white cat trying to tell him?